1. Field of the Invention
This invention relates to an image forming apparatus capable of adjusting the gradation of an image by the use of subsidiary exposure.
2. Description of the Prior Art
Hitherto, in conventional image forming apparatus, the quality of an image being formed was adjusted by gradation adjustment.
Conventional methods of gradation adjustment are to adjust exposure conditions, such as the quantity of light and exposure time of a light source for exposure, to adjust developing conditions, such as the temperature and time of a developing process, to insert various filters into a light path of exposure, to change a light-sensitive material and/or an image receiving material, and so on. However, in view of the cost of forming an image, the price of an image forming apparatus, the efficiency of working, and so on, the method of making use of subsidiary exposure is widely put in practice.
According to one method of adjusting the gradation of an image by the use of subsidiary exposure, a gray (achromatic color) reflecting plate for subsidiary exposure is provided in the vicinity of an original bed mounted on a copying machine of the direct exposure type, this reflecting plate is illuminated by a light source before or after main exposure, and a light-sensitive material is exposed to the reflected light from the reflecting plate whose quantity of light is in a range corresponding to 1/50 to 1/100 of the quantity of light of main exposure.
In Japanese Patent Application Kokai No. 1-303427, a copying machine was proposed which, without using a reflecting plate for subsidiary exposure, performs subsidiary exposure using a white reference plate and a variable stop to adjust the gradation of an image being formed, whereby images of desired gradation are formed.
In the foregoing copying machine, subsidiary exposure or gradation adjustment is performed by decreasing the quantity of light of the reflected light from the white reference plate by means of the variable stop to a level corresponding to 1/50 to 1/100 of the quantity of light of main exposure. Since the variable stop can freely adjust the quantity of light of subsidiary exposure, images of desired gradation are readily copied and formed.
The foregoing copying machine demands no additional expense and can adjust the gradation of an image by simple mechanisms. Since images of any gradation are formed by adjusting the quantity of light by means of the variable stop, the degree of freedom of gradation adjustment is high compared to a machine including a reflecting plate for subsidiary exposure, and images of any desired gradation are readily formed.
In another type of copying machine wherein subsidiary exposure is performed using a gray reflecting plate exclusive to subsidiary exposure, however, the quantity of light of subsidiary exposure is fixed and not changeable; therefore, it is impossible to change the amount of adjustment of gradation in accordance with user's desire to form images of desired gradation.
In the copying machine disclosed in Japanese Patent Application Kokai No. 1-303427, only the quantity of light of subsidiary exposure can be changed linearly within a range corresponding to 1/50 to 1/100 of the quantity of light of main exposure in accordance with the change of gradation; therefore, the gradation can be adjusted to a certain extent. However, even in the type wherein the optimal or standard conditions of color filter and aperture of main exposure are set automatically, the color balance of formed images is distorted depending on the amount of adjustment of gradation, resulting in unnatural images. Therefore, there is obtained no image whose gradation is adjusted as desired.
The scanning exposure type and the one-shot plane exposure type differ in change of gradation even where subsidiary exposure is performed using the same quantity of light, because the strength of light and time of exposure differ between the two types even where the quantity of light impinging on a light-sensitive material is the same. Particularly, even where the quantity of light of subsidiary exposure is the same, the scanning exposure type is weaker in strength of exposure light and longer in exposure time than the plane exposure type, resulting in a difference in, for example, change of gradation among the three primary colors (yellow (Y), magenta (M), cyan (C)).
Therefore, to always form fine images of good color balance irrespective of the amount of adjustment of gradation caused by subsidiary exposure, images must be actually formed from an original picture or test chart using subsidiary exposure; color filters (yellow (Y), magenta (M), cyan (C)) and a stop must be adjusted manually at the time of subsidiary exposure while observing the thus obtained hard copies of images; and the foregoing steps must be repeated to set the optimal conditions of subsidiary exposure.
Further, even where the optimal conditions of color filter and aperture of subsidiary exposure are set, if the characteristics of light-sensitive materials and/or image receiving materials, the property of images, the characteristics of developing/fixing solutions, the heat developing temperature, the processing conditions, and so on change, the optimal conditions of subsidiary exposure change even after adjusting the color and quantity of light of main exposure, thereby resulting in unnatural images of distorted color balance.
A further type is known in which the optimal or standard conditions of subsidiary exposure are adjusted independent of main exposure. However, since the standard conditions of main exposure are obtained independent of those of subsidiary exposure, complicated image adjustment must be performed.